The present invention is a method for recovering bitumen from aqueous streams containing bitumen and mineral matter. In one aspect the present invention is an improvement in the hot water process for extracting bitumen from tar sands. Specifically, the present invention is a method whereby bitumen can be recovered from aqueous streams such as tar sands pulp, effluent discharge streams from a hot water process for extracting bitumen from tar sands or retention pond water associated with tar sands processing.
Tar sands, which are also known as oil sands and bituminous sands, are impregnated with a heavy petroleum. The largest and most important deposits of the sands are those found in northern Alberta, Canada, known as the Athabasca sands. The Athabasca sands are primarily silica, having closely associated therewith an oil film which varies from about 5 percent to 21 percent by weight, with a typical content of 13 weight percent of the sand. The bitumen is quite viscous--6.degree. to 8.degree. API gravity-- and contains about 4.5 percent sulfur and about 38 percent aromatics.
The sands contain in addition to the oil and sand components, clay and silt in quantities of from 1 to 50 weight percent, more usually 10 to 30 percent. The sands also contain a small amount of water, in quantities of 1 to 10 percent by weight, in the form of a film around the sand grains.
Several basic extraction methods have been known for years for recovering oil from these sands. The most widely known procedure is the hot water extraction method disclosed in Canadian Patent No. 841,581 issued May 12, 1970, to James Van Dyck Fear et al.
Other proposed methods for recovering bitumen from tar sands include the so called "cold water" method, which involves mixing tar sands with a solvent capable of dissolving the bitumen constituent. The mixture is then introduced into a large volume of water containing a surface agent or an inorganic salt capable of acting as an electrolyte. The combined mass is then subjected to a pressure or gravity separation.
U.S. Pat. No. 2,965,557 issued Dec. 20, 1960 to W.H. Price discloses a method for recovering bitumen from tar sands which comprises mixing the sand with a hydrocarbon diluent and thereafter introducing gas into the sand-diluent mixture to effect separation of the bitumen from the sand prior to settling the sand in an aqueous settling zone.
U.S. Pat. No. 3,203,888 issued Aug. 31, 1965 to R.M. Butler et al. proposes a method for recovering bitumen from tar sands wherein a mixture of tar sands and water is blended with liquified hydrocarbon gas at a pressure sufficient to maintain the gas in a liquid state. The gas-water-tar sands mixture is thereafter agitated and then settled at a lower pressure whereby the added liquid hydrocarbon vaporizes to aid in flotation of bitumen. A critical element of this invention is the solubility of the added liquid hydrocarbon in the bitumen tar sands to provide effective separation of the bitumen constituent from the sand particles.
U.S. Pat. No. 3,573,195 issued Mar. 30, 1971 to C.W. Bowman et al. discloses a method for recovering bitumen from tar sands whereby a liquid hydrocarbon is added to an aqueous slurry of tar sands at a temperature of less than 110.degree.F. Thereafter the mixture is settled in a settling zone maintained above 150.degree.F. An essential element of this invention is that the added liquid hydrocarbon boils above 110.degree.F. so that when the mixture is heated, e.g., to 150.degree.F., the hydrocarbon vaporizes to aid in the flotation of the bitumen in the mixture.
U.S. Pat. No. 3,574,086 issued Apr. 6, 1971 to A.W. Hyndman proposes a method for recovering bitumen from tar sands wherein an aqueous slurry of tar sands is formed in an atmosphere of a gaseous hydrocarbon so that the gas is entrained in the slurry and subsequently aids in flotation of the bitumen component of that slurry in a settling procedure which follows.
Canadian Pat. No. 882,666 issued Oct. 5, 1971 to Harold F. Tse proposes forming a mixture of tar sands, water, solvent (boiling at a higher temperature than water) at a temperature in the range of 220.degree. to 325.degree.F. and a pressure between 16 and 100 psig; passing the mixture to a zone having temperature and pressure conditions to provide that at least a part of the water vaporizes as steam to aid flotation of froth; and subsequently recovering bitumen froth.
In the hot water method, as disclosed in Canadian Pat. No. 841,581 issued May 12, 1970, the bituminous sands are jetted with steam and mulled with a minor amount of hot water at temperatures of 170.degree. to 190.degree.F., and the resulting pulp is then dropped into a turbulent stream of circluating hot water and carried to a separation cell maintained at a temperature of about 185.degree.F. In the separation cell, sand settles to the bottom as tailings and oil rises to the top in the form of a froth. An aqueous middlings layer comprising clay and silt and some oil is formed between these layers. This basic process may be combined with a scavenger step for further treatment of the middlings layer obtained from the primary separation step to recover an additional amount of oil therefrom.
The middlings layer either as it is recovered from the primary process or as it is recovered after the scavenger step comprises water, clay and oil. The oil content is of course, higher in middlings which have not undergone secondary scavenger steps.
Hereinafter in this specification, the term "effluent discharge" will be used to describe any aqueous stream associated with the hot water extraction of bitumen from tar sands which stream is not the primary product of the process. These streams include unprocessed middlings material, middlings material of depleted oil content which has undergone final treatment and which comprises clay dispersed in water, the sand tailings layer also containing some clay and bitumen and other discharged water-containing fractions, which are recovered as waste products. The effluent discharge is removed from the process plant as a slurry of about 35 to 55, typically 45 percent, solids by weight. Included in the slurry is sand, silt, clay and small quantities of bitumen. In this specification, sand is siliceous material which will not pass a 325 mesh screen. Silt will pass 325 mesh but is larger than 2 microns. Clay is material smaller than 2 microns including some siliceous material of that size. Included in the slurry is sand, silt, clay and small quantities of bitumen ranging from about 0.5 to 2.0 weight percent of the total discharge.
Because the effluent contains oil emulsions, finely dispersed clay with poor settling characteristics and other contaminants, water pollution considerations prohibit discarding the effluent into rivers, lakes or other natural bodies of water. The disposal of the effluent discharge has therefore presented a problem. Currently, effluent discharge is stored in evaporation ponds which involve large space requirements and the construction of expensive enclosure dikes. A portion of the water in the effluent discharge is recycled back into the hot water extraction process as an economic measure to conserve both heat and water. However, experience has shown that the dispersed silt and clay content of the recycled water can reduce primary froth yield by increasing the viscosity of the middlings layer and retarding the upward settling of oil flecks. When this occurs, the smaller oil flecks and those that are most heavily laden with mineral matter stay suspended in the water of the separation cell and are removed from the cell with the middlings layer.
Effluent discharge from the hot water process for extracting bitumen from tar sands as generally disclosed contains a substantial amount of mineral matter, much of which is colloidally dispersed in the effluent discharge and therefore does not settle very readily when stored in the retention pond. The lower layer of the retention pond can contain up to 50 percent dispersed mineral matter substantially of clay and silt as well as up to 5 percent bitumen. This part of the pond water is normally referred to as sludge. Sludge is not suitable for recycling to the hot water extraction process for the reason that its addition into the separation cell or the scavenger cell at the normal inlet means would raise the mineral content of the middlings of the cell to the extent that recovery of bitumen would be substantially reduced. Generally, the settling which does take place in the pond provides a body of water in which the concentration of mineral matter increases substantially from the surface of the pond to the bottom thereof. As a typical example a pond of effluent discharge having a surface area of about 1000 acres and an average depth of 40 feet can be characterized somewhat as follows:
a. From the surface of the pond to a depth of 15 feet the mineral concentration which is primarily clay is found to be about 0.5 to 5.0 weight percent. This pond water can normally be recycled to a hot water extraction process without interfering with the extraction of bitumen from tar sands.
b. The layer of water in the pond between 15 and 25 feet from the surface contains between 6 and 15 percent mineral matter. This water if recycled to the separation cell feed with fresh tar sands would increase the mineral content of the middlings portion of the cell to the point that little bitumen would be recovered.
c. Finally, the section of the pond between 25 feet and the bottom of the pond contains 16 to 50 percent mineral matter and is normally referred to as sludge.
The present invention provides a method for recovering bitumen from any aqueous stream containing the same. More specifically, the present invention comprises a method whereby recovery of bitumen from any of the aqueous bitumen-containing streams associated with a hot water process such as that noted above can be improved. In one aspect the present invention can be considered an improvement in the hot water process for extracting bitumen from tar sands.